1. Field of Invention
The present invention relates to a rotary encoder used for detecting a rotating angle of a steering in a power steering system and the like of a vehicle, and in particular, relates to a mounting structure of a rotor placed therewithin.
2. Description of the Prior Art
A conventional rotary encoder R1 will be described with reference to drawings. FIG. 9 is a cross-sectional view of main parts of the conventional rotary encoder, FIG. 10 is a plan view of a speed nut thereof, and FIG. 11 is an enlarged cross-sectional view of main parts of the conventional rotary encoder.
Now, referring to FIGS. 9 to 11, a structure of the conventional rotary encoder will be described. To a main body 100 to which the rotary encoder R1 is to be attached, a cylinder 102 is formed in such a manner that the cylinder 102 projects from a part of a wall 101 in a bottomed cylindrical shape. In the cylinder 102, a bearing 103 composed of a ball bearing having a plurality of balls is disposed.
In the center of a sidewall of the cylinder 102, a through hole 104 in a circular shape is formed.
The rotary encoder R1 includes a casing 110 composed of a double-stack tube portion, a shaft 120, a code plate 130 as a code member rotating together with the shaft 120, a hall element 113 disposed opposite to the code plate 130, and a speed nut 140 for holding the code plate 130 so as not to fall off from the shaft 120.
The casing 110 is formed so that the cross section thereof has a generally T-letter shape, and an opening 111 is formed on the sidewall thereof (on an upper side of FIG. 9). On an opening side of the cylinder is fixed to the main body 100 by an appropriate means such as soldering. A printed circuit board 112 of a rectangular flat plate is attached to the opening 111 so as to project outward. To the printed circuit board 112, the hall element 113 is attached while being connected to a detection circuit (not shown) or the like, and the hall element 113 is placed at the opening 111.
The shaft 120 includes a first shaft 121 having a larger diameter, and a second shaft 122 having a smaller diameter formed in such a manner that it projects from an end of the first shaft 121. The first shaft 121 is rotatably held by the bearing 103 through a through hole 104 of the cylinder 102, and the second shaft 122 is located inside the casing 110 while projecting from the cylinder 102.
The code plate 130 is formed of a permanent magnet in a disk shape, and a through hole 131 is formed at the center thereof. The second shaft 122 is loosely fitted into the through hole 131 of the code plate 130, and one side of the code plate 130 is in contact with a tip surface of the first shaft 121 as a step placed on a border of the first and the second shaft 121 and 122, respectively.
A speed nut 140 is composed of a metal disk plate, and as shown in FIG. 10, it has a base body 141 in a disk shape, and three tongues 143 formed so as to project from the center of the base body 141 by providing notches 142 on both sides thereof, and a through hole 144 in a disk shape formed in the center of the base body 141. The speed nut 140 with such structure is fitted tightly into the second shaft 122, and pressed on another surface of the code plate 130 so as to prevent the shaft 120 from falling off from the code plate 130. Specifically, the second shaft 122 is inserted into the through hole 144, and the base body 141 is pressed toward the first shaft 121, and the code plate 130 is interposed between the speed nut 140 and the first shaft 121. Each of the three tongues 143 is bent toward an opposite direction from the pressing direction (toward the left in FIG. 9) so as to be tightly fitted into the second shaft 122. By tightly fitting the tongue 143, the speed nut 140 cannot be moved in a direction of an axial line, and thus, the code plate 130 is prevented from falling off, and the speed nut 140 and the code plate 130 rotate together with the shaft 120.
Because the second shaft 122 is loosely fitted to the through hole 131 of the code plate 130, as shown in FIG. 11, the center C1 of the through hole 131 and the center C2 of the second shaft 122 are shifted in a direction orthogonal to the axial line direction, whereby positions of the second shaft 122 and the code plate 130 are determined by the speed nut 140.
Next, operation of the conventional rotary encoder R1 will be described. As the shaft 120 rotates via a steering shaft (not shown), the code plate 130 rotates therewith. The hall element 113 detects a change of magnetic pole so that a magnetic detection circuit (not shown) formed on the printed circuit board 112 detects a detection pulse corresponding to rotation of the code plate 130. When the shaft 120 rotates while the center C1 of the through hole 131 and the center C2 of the shaft 122 are shifted from each other, the code plate 130 rotates so as to represent an elliptical contour, whereby accurate pulse detection cannot be achieved. Moreover, a detected value becomes different for every rotary encoder R1, and thus, there is a concern for not being able to measure rotating angle accurately.
In the conventional rotary encoder R1 described above, the speed nut 140 is fitted into the second shaft 122 while being positioned at an end of the code plate 130, i.e., it is simply in contact with the end of the code plate 130. Therefore, there is a possibility that positions of the speed nut 140 and the code plate 130 may be shifted from each other. If such shifting occurs between the speed nut 140 and the code plate 130, the center positions C1 and C2 of the speed nut 140 and the code plate 130, respectively, are not determined, and thus, when the shaft 120 rotates, the code plate 130 rotates as it represents an elliptical contour, whereby its performance deteriorates. In order to prevent such shift, precision of the through hole 131 of the code plate 130 and the second shaft 122 may be corrected manually, but it would lead to problems such as a deteriorated mass productivity and an increased cost. Moreover, when precision of the size of both parts is enhanced, the cost would increase.
The present invention has been achieved in view of the above problems, and its object is to provide a rotary encoder whose code member moves along an accurate circle as a shaft rotates.
In order to solve the above-described problems, in one embodiment, a rotary encoder according to the present invention has a structure, including a casing; a detection element attached to the casing; and a rotatable rotor having a mounting aperture for inserting a shaft at the center thereof and being provided with a code member opposed to the detection element, in which an inner peripheral surface of the mounting aperture is provided with a spring member having a base and a plurality of tongues for energizing the shaft toward the center of the mounting aperture so as to set a position of the shaft at the center of the mounting aperture.
In a second embodiment, the spring member of a rotary encoder according to the present invention is formed from a metal plate, the base of the spring member is in a plate shape, and the tongue is formed in series to the base so as to project in a direction of a rotation axis line of the rotor, the tongue being in elastic contact with the shaft at generally mid-portion in a direction of a rotation axis line of the mounting aperture.
In a third embodiment, the tongue of a rotary encoder according to the present invention is provided with a flat portion parallel to the rotation axis line of the rotor and being in elastic contact with an outer periphery of the shaft.
In a fourth embodiment, the tongue of a rotary encoder according to the present invention is supported at both ends thereof by the base.
In a fifth embodiment, the tongue of a rotary encoder according to the present invention is supported at one end thereof with respect to the base.
In a sixth embodiment, the tongue of a rotary encoder according to the present invention has at least one pair of projections formed to be disposed in a direction of the rotation axis line of the rotor, the projections projecting toward the center of the mounting aperture and being in contact with the outer periphery of the shaft.
In a seventh embodiment, the code member of a rotary encoder according to the present invention is made of a magnet, and a basal portion of a rising portion of the tongue notched up from the base is disposed at a position out of a thickness range in a direction of the rotation axis line of the code member.
In an eighth embodiment, the base of the spring member of an rotary encoder of the present invention is formed by bending into a cylindrical shape, and is housed within the mounting aperture of the rotor.
Moreover, in a ninth embodiment, the spring member of a rotary encoder according to the present invention is formed from a square plate material.
Furthermore, in a tenth embodiment, the spring member of a rotary encoder according to the present invention has an engaging portion projecting from an end thereof by bending generally perpendicularly, the engaging portion being latched by an end of the rotor.